Jean-Marc Lefebvre

Plastic deformation behaviour of thermoplastic/clay nanocomposites

Abstract Polymer/clay nanocomposites are materials that display rather unique properties, even at low clay content, by comparison to more conventional mineral-filled polymers. Two systems are considered in the present study: the first one consists of nylon 6/clay hybrids in which in situ polymerisation is aimed at obtaining a nylon matrix strongly bonded to the delaminated clay platelets. The second one is prepared by melt dispersion of organophilic clay in polypropylene, which should in principle result in a reduced degree of polymer–clay interaction. Dynamic viscoelastic analysis is indeed indicative of a noticeable difference when referring to the molecular dynamics of the glass transition. Plasticity results, in which volume strain is recorded by video-extensometry, show extensive cavitational behaviour while retaining a fairly large strain at break, as long as deformation is performed above the glass transition temperature of the matrix. In the particular case of PA6, it is clear that the usual shear banding plastic deformation mode is altered at least in its initiation step. Localised interfacial damage promotes extensive polymer matrix fibrillation and fracture occurs predominantly in areas where delamination of the clay platelets was not fully achieved.

Structural and mechanical behavior of nylon 6 films part I. Identification and stability of the crystalline phases

The crystalline phase of polyamide 6 (otherwise nylon 6) films produced following various physical treatments is investigated by means of thermal analysis, X-ray diffraction, and infrared spectroscopy. A recently published procedure for treating infrared spectra of multicomponent compounds without a priori knowledge of the individual component spectra allowed us to perform a semiquantitative analysis of the structural changes upon annealing, including data from drawn samples. Melt-cast films display a mesomorphic state that is thermally stable up to about 120 °C. This structure partly reorganizes into the stable monoclinic α form above 120 °C. Films in major γ form produced by iodine treatment are thermally stable up to 200 °C. Films in major α form are also prepared by superheated water treatment. No evidence is given for a Brill transition about 170 °C. This is an important fact for further understanding of the drawing behavior of PA6 at temperatures above and below this domain. The mesomorphic phase can hardly be separated from the amorphous component both from X-ray and infrared analysis. However, scanning calorimetry, which is often criticized due to possible reorganization of unstable species during the heating scan, turned out to be a very useful technique. Indeed, recrystallization from the amorphous phase or improvement of ordering from the mesomorphic state both result in exothermic effects in quite different temperature domains that allow to discriminate the two phenomena.

Plastic deformation of glassy amorphous polymers: influence of strain rate

The compressive deformation of glassy atactic polymethyl methacrylate has been studied in the temperature range 150<T<330 K as a function of applied strain rate. Special emphasis is given to the definition of an elementary activation rate, i.e. the characteristic frequency of the successful plastic deformation events. This determines the frequency at which the elastic counteraction of the medium to the nucleation of plasticity has to be taken into account, in order to ensure self-consistence of the kinetic and thermodynamic analysis of deformation previously proposed.

Tensile drawing of ethylene/vinyl-alcohol copolymers. Part 1. Influence of draw temperature on the mechanical behaviour

The uniaxial tensile drawing of two vinyl-alcohol-rich ethylene/vinyl-alcohol copolymers differing in composition and melt flow index is studied as a function of draw temperature. A change of mechanical properties occurs in the strain range of the strain-hardening threshold depending on whether the draw temperature is above or below a critical value of about 100°C. A strong propensity for necking and concomitant longitudinal fissuration of the films take place above this temperature for the copolymer of lower vinyl-alcohol content. The vinyl-alcohol richer copolymer, having a higher melting point and a lower melt flow index, exhibits similar trends at higher temperatures. Bulk materials also display longitudinal fissures originating from transverse craze-like defects located along the neck shoulder. It is suggested that the drastic weakening of the van der Waals (v.d.w.) interactions of paraffinic nature between the hydrogen-bonded sheets of the monoclinic crystal structure, above 100°C, results in a strong mechanical anisotropy. This may perfectly account for the fissuring trend thanks to the easy glide of the weakly interacting sheets. In contrast, a strain-induced disorganization of the crystalline structure triggers improved drawability to the copolymers at low draw temperature, owing to the mechanical isotropy of the new mesomorphic structure type.

Plasticity of rubber-toughened poly(methyl methacrylate): effect of rubber particle size

The plastic deformation of rubber-toughened poly(methyl methacrylate) has been investigated in compression at constant strain rate as a function of particle volume fraction and particle size. The behaviour at yield appears insensitive to particle size for particle diameters d in the range 80 nm < d < 300 nm. In contrast, work-hardening rate measurements in the pre-yield region reveal an abrupt change in the materials ability to develop plasticity. This transition from difficult to easy shear-band formation as the rubber volume fraction is varied is shown to occur for the same surface-to-surface interparticle distance, whatever the particle size.

Thermally activated deformation of glassy polystyrene

Abstract The deformation of polystyrene in compression at constant strain rate was studied in the temperature range 150 – 330 K. Thermal pretreatments of the samples to prevent possible discrepancies in compression tests are discussed. The experiments were analysed in terms of thermally activated processes by using the activated slip analysis for crystalline solids. The activation parameters and particularly the entropy contribution were measured in order to estimate the free energy associated with the deformation mechanism. The results show that two different deformation modes should be considered, depending on temperature, above and below some critical temperature near 280 K. It was established that at lower temperatures the stress dependence of the strain rate is purely exponential, i.e. there is a true stress activation volume; its value does not vary much and corresponds to a coherent fluctuation of about five or six monomers. At higher temperatures, the stress contributes to the strain rate probably only in the pre-exponential factor in the form of some power law; moreover, the activation energy seems to be lower than in the low temperature mode.

Constitutive modelling of the large inelastic deformation behaviour of rubber-toughened poly(methyl methacrylate): effects of strain rate, temperature and rubber-phase volume fraction

A combined approach including experimental investigation and constitutive modelling was followed in this work to study the stress–strain behaviour of rubber-toughened glassy polymers. The large inelastic deformation response of rubber-toughened poly(methyl methacrylate) (RT-PMMA) was experimentally studied under uniaxial compression tests at different strain rates and temperatures. The studied composite system consists of spherical core–shell (PMMA hard shell and soft rubber core) particles embedded in a PMMA matrix. The influence of particle concentration (ranging from 0% to 45%) on the macroscopic behaviour was also investigated from small to large strain. The physically based hyperelastic–viscoplastic constitutive model of Boyce–Socrate–Llana was extended to describe the stress–strain behaviour of rubber-toughened glassy polymers. The model accounts for the effective contribution of the two polymeric phases to the overall composite macroscopic behaviour, by including in the original model the hyperelastic deformation of rubber particles. The capabilities of the model to describe the rate-dependent yield and post-yield behaviour of PMMA over a wide range of temperatures and strain rates are pointed out. The model is able to successfully capture the significant features of the stress–strain behaviour including the initial linear elasticity, the gradual rollover to yield, the strain softening after yield (when it exists) followed by the strain hardening. Its predictive capabilities are further tested by comparison with compression data on RT-PMMA for different rubber contents.

Work-hardening rate of glassy polymers: evolution with curing of thermo-set resins

A new parameter, the work-hardening rate,K, is introduced to characterize the non-elastic deformation behaviour of glassy polymers. Related to the defects nucleation in the preyield stage, this parameter is shown to be a very sensitive probe of the structural evolution of materials. The variation ofK with cross-linking in the curing of a polyimide resin is reported.

The preyield evolution with strain of the work-hardening rate in glassy polymers (PABM resin)

The work-hardening rate K, measured in the early preyield stage of constant strain-rate compression tests, is found to vary as the inverse of the non-elastic strain ɛp. From a metallurgical point of view, we show that, in the early stage of development of thermally activated glide processes, this behaviour can be predicted from simple assumptions on the shear nuclei kinetics. The ɛp-dependence of K is tested in the case of a tightly cross-linked polyimide polyamino-bismaleimide, PABM, resin.

Tensile drawing of ethylene/vinyl-alcohol copolymers: 3. Biaxial orientation

Abstract The phenomenological and structural aspects of biaxial drawing of a vinyl alcohol-rich ethylene/vinyl-alcohol copolymer is studied as a function of temperature. For draw temperature above the crystalline mechanical relaxation, T α , sharp necks accompanied with catastrophic fissures develop during simultaneous biaxial drawing as well as in the first step of sequential biaxial drawing. For draw temperature below T α , simultaneous biaxial drawing can be successfully achieved via propagation of diffuse necking so that a minimum macroscopic draw ratio of about 3 × 3 is required to obtain a uniform plastic strain. The strong interaction anisotropy of the sheet-like structure of the monoclinic phase is responsible for the fissuring trend at T > T α . In contrast, the isotropic mesomorphic phase that is strain-induced at T T α enables simultaneous biaxial drawing. The development of a planar texture suggests that the transformation results from transverse crystal slip. However, the random distribution of the H-bonds in the mesomorphic phase is rather consistent with an accumulation process of conformational defects that remain frozen in the crystal.